The present invention relates to miniature electrically insulated multi-conductor electrical cables, particularly those which may be implanted in living persons, and methods for making such cables.
Many miniaturized electrical sensors and control devices may conceivably be implanted in human bodies or may be used in robotic devices. Such sensors and other devices may require reliable transmission of electrical signals, yet may have extremely small current requirements. In many cases flexibility of the conductors is highly desired. For example, electrical signals must be transmitted between an implanted biomedical sensor and an electronically controlled device such as a drug delivery device implanted in a separate location. It may also be desirable to provide electrical stimulation to, or to receive signals from, several separate neurons of the human nervous system, at sites spaced as closely as 100 microns from one another. For example, extremely small biologically compatible cochlear implant devices have been designed which utilize multi-electrode arrays to selectively stimulate portions of the basilar membrane to induce hearing in compromised individuals. It may also be desired to provide functional electrical stimulation by surface electrodes or implanted electrodes to mobilize paraplegics. The smaller and more flexible the cable implanted, the less it is likely to be an irritant to the surrounding tissue.
Ultimately, long term implantation of some biomedical electrical devices may be possible, allowing great advances in diagnostic and remedial medicine. However, a major limiting factor in the usefulness of such devices is the lack of suitable conductors for transmission of electrical energy for device actuation and diagnostic signal transmission. Complications in the desired in-situ application of such electrical conductors are the requirements for flexibility, extremely small size, long-term reliability, and biological compatibility of materials implanted.
It is known to utilize photolithographic techniques to provide flex circuits of fine line noble metal conductors supported by flexible polyimide substrates. A distinct disadvantage of such flexible circuit technology is the limitation of flexibility to a single plane. Additionally, such flex circuits can typically be made only in limited lengths. There is also some concern about the biological compatibility with implantation of such flex circuits because of the materials and chemicals used in the process of their production.
Keane, et al. U.S. Pat. Nos. 4,503,124 and 4,537,804 disclose a conductor insulated with a polyimide overcoated with polyesterimide. The purpose of the Keane coating system, however, is not to provide an implantable cable, and there is no teaching in the Keane et al. patents relating to the formation of terminal portions which can be connected easily to printed circuits.
Haley et al. U.S. Pat. No. 4,819,329 discloses making a cable by holding the terminal portions of individual conductors in a fixture, with the ends of the wires stripped of their normal insulating coating to facilitate connection of the wires to printed circuits or the like. The middle portions of the wires can then be insulated and may be twisted together as a cable.
Handa et al. U.S. Pat. No. 4,964,414 discloses an electrode for implantation. Fine stainless steel wires of less than 25 micron diameter are twisted together and coated with a resin which retains the wires and allows them to be implanted.
Praeger et al. U.S. Pat. No. 3,751,801 discloses a miniature ribbon cable whose insulation is removed from end portions of the conductor wires. Alternate wires are bent in opposite directions to form two rows for soldering to a terminal, at a pitch established by the ribbon cable.
Folk U.S. Pat. Nos. 3,874,077 and 3,881,246 are closely related to each other and disclose the use of grooved templates to receive the several conductors of a cable so that the ends of the conductors may be cut off evenly to receive terminals. However, there is no teaching of how to prepare the conductors to facilitate connection of a very small cable to a printed circuit.
Rich U.S. Pat. No. 4,614,028 discloses a fixture for use in changing the pitch of the conductors of a multi-conductor flat ribbon cable. There is no teaching of how to hold the conductor ends in a desired pitch, however, and the apparatus is not directed to a small cable intended to be implantable.
Lekholm, et al. U.S. Pat. No. 4,840,186 discloses a lead in which multifilament conductors coated with insulation are embedded in a tube wall in a helical arrangement, but the lead so produced is much larger and would be less flexible than desired for certain cable implantations.
Compte U.S. Pat. No. 4,640,983 discloses an electrical conductor intended for implantation into humans, in which multifilament conductors are provided in spiral form and sheathed in a biologically compatible material. The conductor disclosed, however, is much larger than the desired implantable cable with which the present invention is concerned.
What is desired, then, is a multi-conductor electrical cable of extremely small size which can be implanted in a living body without causing continuing irritation, and which is flexible, durable, electrically insulated, and capable of readily being connected electrically to extremely closely spaced terminals.